Use of global positioning satellite (GPS) receivers in automotive navigation, emergency messaging, and tracking systems is now widespread. However, systems based solely on GPS generally do not work well in dense city environments, where signal blockage and reflection by tall buildings, in addition to radio frequency interference, often occurs. A cost effective solution to this problem is to augment the GPS receiver with some form of Dead Reckoning (DR), to fill in the gaps occurring as a result of loss of GPS coverage and improve the accuracy of the GPS trajectory.
A DR system may take the form of an interface to separate left and right wheel sensors installed in the vehicle to provide an indication of the speed of each wheel. The average speed of each wheel is used to determine the vehicle velocity, and the wheel speed difference divided by the distance between the wheels (referred to as the wheel track) is used to determine changes in the vehicle heading. The accuracy of the DR system is critically dependent upon the accuracy to which the vehicle's heading is determined whereby each degree of heading error, in the absence of GPS, produces a cross-track position error which grows approximately as 1.7% of distance traveled.
However, in order to derive accurate heading information from the speed of each wheel, the distance between the centers of the tires, or the wheel track, must be known to high precision whereby an error of 1% is generally acceptable, since it produces roughly one degree of heading error each time the vehicle turns a corner (i.e., changes its heading by ninety degrees). Unfortunately, the wheel track may not always be known to this level of accuracy. Further, different classes of the same vehicle type can produce variations of up to 5%, which produces significant navigation error when GPS positioning is lost. For example, a 5% error in the wheel track will produce nearly ten meters of cross track position error every one hundred meters of travel following a reversal of the vehicle's direction (i.e., a one hundred eighty degree turn). Thus, the wheel track must be known to a high level of precision for each vehicle in which the wheel sensor based DR system is installed. This places an undesirable burden on the vehicle manufacturer, since the DR system must be informed of the vehicle type and class, or have the wheel track value input directly.
Thus, a method of estimating the wheel track of a vehicle to a high level of accuracy is needed.